An industrial scale process utilizing hexacyanoferrate-based ion exchangers was developed for the selective separation of radioactive cesium from nuclear waste solutions. This process was put into operation at the Loviisa Nuclear Power Plant (NPP) (pressurized water reactor, VVER-440), Finland, at the end of 1991, and it has shown superiority to any other cesium separation method used at present at nuclear plants. This paper summarizes the work that was carried out in the development of this process. In the first phase of the work, the performance of several cesium-specific precipitants and ion exchangers (eg., zeolites and hexacyanoferrates) was tested by laboratory experiments. Based on these initial tests, two precipitants, sodium hexanitrocobaltate and tungstophosphoric acid, and two hexacyanoferrate exchangers were chosen for pilotscale experiments. These experiments showed that the hexacyanoferrate ion exchangers were the most efficient materials for the removal of 137Cs and 134Cs and were suitable for large-scale column operation. With hexacyanoferrates, decontamination factors (DFs) of several thousands and volume reduction factors (VRFs) in the range of 2000 to 10000, were obtained for 137Cs and 134Cs. By using the cesium-specific precipitants, DFs and VRFs on the order of 100 were feasible in the Loviisa concentrates. After the pilot experiments, an exchanger based on hexacyanoferrate was chosen to be used in the full-scale cesium-separation plant constructed at the Loviisa NPP.